Fuel cell systems where oxygen is supplied from ambient air accumulate the non-reactive components of air (primarily nitrogen and some water vapor or condensation) in the fuel stream due to finite diffusion rates of gases through the fuel cell electrolyte. The inert gas accumulation eventually lowers the fuel cell output voltage due to drop of fuel concentration. As a consequence, continuous operation requires periodic purging of the fuel compartment. Additionally, fuel cell systems often employ safety valves that allow gas to escape if the internal pressure or vacuum builds to unsafe levels, preventing damage to the device and/or hazards to users. Two types of methods for addressing these issues include active and passive purge valves. In active purge systems, an electrically or mechanically controlled valve is employed at the outlet of the fuel gas flow path to allow the fuel and accumulated nitrogen to escape when necessary. In smaller micro-fuel cell systems, miniature valves are often used when minimum size and weight is desired, such as the X-Valve available from Parker Hannefin. These active valves suffer from a number of problems including high cost and high power consumption. Additionally, they are unreliable as a safety purge valve, as they require proper external control in order to function properly. Passive purge valve systems allow gas pressure or vacuum to be released at a specified pressure. Accumulated non-reactive gases can be purged by increasing the operating pressure of the system above the purge pressure of the valve, allowing gas to escape. These valves tend to be less expensive than active valves and do not require external control, making them more reliable. These passive valves include poppet valves, like those available from Smart Products and duck bill valves, like those available from Vernay. Nevertheless a purge system that is based on passive valves requires a good control of the pressure upstream of the purge valve to avoid fuel loss as well as excessive purging. In many hydrogen fuel cell systems, for example, hydrogen is generated on demand such as using binary chemical reactions. The response time of such systems is often characterized by latency and long time constants that are due to finite thermal mass and mass transfer limitations of the chemical hydrogen reactor systems. These limitations make frequent rapid pressure changes impossible and thus purging based on passive purge valves impractical.
Additionally, the current fuel cell systems often operate with hydrogen stored at elevated pressures, requiring high pressure rated gas routing, as well as down pressure regulators, that add system weight. Furthermore, high pressure hydrogen gas routing poses safety challenges in fault modes.
Accordingly, there is a need to develop a fuel cell control scheme that allows for active control of H2 pressure that minimizes safety risks, system complexity, and system weight, while it maintains high fuel utilization.